1. Field of the Invention
This invention relates to catalytic electrodes for electrochemical cells.
2. Description of the Prior Art
The term "fuel cell" is used herein and in the art to denote a device, system, or apparatus in which the chemical energy of a fluid combustible fuel such as hydrogen, carbon monoxide, or an organic compound containing hydrogen in its molecular structure is electrochemically converted to electrical energy at a nonsacrificial or inert electrode. A complete fuel cell is adapted for continuous operation and is supplied with both fuel and oxygen from sources outside the cell proper. Such cells include at least two nonsacrificial or inert electrodes, functioning as an anode and a cathode, respectively, which are separated by an electrolyte which also provides ionic conductance therebetween. There is also provided conduction means for electrical connection between anode and cathode external to the electrolyte, means for admitting a liquid or gaseous fuel into dual contact with the anode and electrolyte, and means for admitting oxygen in dual contact with the cathode and electrolyte. The electrolyte compartments in conventional fuel cells are divided into an anolyte and a catholyte compartment by an ion permeable partition or ion exchange membrane, such as in the solid polymer electrolyte, also known as a proton exchange membrane (PEM) type fuel cell. In this PEM type cell, the membrane, which is a solid polyelectrolyte, acts both as the electrolyte and the cell separator, thus requiring no additional electrolyte. This cell is also termed an ion exchange membrane type fuel cell. In each such cell, a fuel is oxidized at the anode and an oxidant is reduced at the cathode upon receiving electrons from the anode.
Electrodes of the type hereinbefore and hereinafter described are also employed in electrolytic cells which unlike the aforementioned fuel cells do not provide a net production of electrical energy, but in which an organic fuel is oxidized electrochemically at the anode thereof. In such cells, a direct current of electrical energy from an external source, namely a fuel cell, a storage battery or an alternating current rectifier, is admitted to the electrical circuit of the cell to provide the necessary electrical current to operate the cell. Such cells can be used for the electrochemical production of various organic chemicals, such as the conversion of alcohols or hydrocarbons to ketones.
Electrodes for use in these cells vary considerably in both design and composition. Although a single metal structure, such as a platinum sheet or screen, or a structure of porous carbon, such as a flat sheet or a porous carbon cylinder, can be used alone, electrodes commonly comprise a conductive base or current collector with a metal catalyst chemically and/or physically bound to the surface of the base. Such electrodes also include those upon which the catalyst is laid down by electro-deposition, and those which are impregnated with catalyst by soaking the base in a solution comprising a suitable catalyst yielding material, decomposing the adsorbed material and/or reducing the resulting metal-containing material to elemental metal or metal oxide. The latter technique is conventional in the preparation of porous carbon electrodes bearing a metal catalyst. Noble metals, particularly platinum, are effective catalysts in both oxidation-reduction reactions wherein either a basic or acid electrolyte is employed in the cell.
The use of monomeric, as well as, polymeric metal phthalocyanine compounds as oxidation catalysts for chemical reactions are known. For example, nickel phthalocyanine has been employed in the oxidation of long-chain fatty acids, esters, saturated ketones, benzene hydrocarbons, etc. Such catalysts, particularly cobalt phthalocyanine, when used as an active component in the cathode of a fuel cell are advantageous over known electrode catalysts comprised of noble metals, primarily in that the cobalt phthalocyanine catalyst is relatively inexpensive and can be produced in any desired amount. One disadvantage of electrodes comprising cobalt phthalocyanine is that this compound has an extremely low conductivity in comparison with noble metal catalyst compositions and, therefore, such metal phthalocyanine catalyst must be applied in very thin layers upon the surfaces of conducting carrier material in the preparation of electrodes. Electrochemical cells having electrodes comprising cobalt phthalocyanine, are disclosed in U.S. Pat. No. 3,585,079 and U.S. Pat. No. 4,255,498. Heat treated, carbon supported metalloporphyrins and metallophthalocyanines as oxygen reduction catalysts are disclosed in J. Chem. Society; Faraday trans. 1, 77, 2827-2843 (1981).
It is conventional to prepare electrodes for electrolytic cells by mixing powdered or granular active carbon particles which act as a carrier or support material for the adsorbed catalyst layer. Such electrodes are prepared by mixing the catalyzed carbon particles with a water-repellent binder such as polytetrafluoroethylene and compressing the mixture into a thin sheet.